Abiotic stress is the primary cause of crop loss worldwide, causing average yield losses of more than 50% for major crops (Boyer, J. S. (1982) Science 218:443-448; Bray, E. A. et al. (2000) In Biochemistry and Molecular Biology of Plants, Edited by Buchannan, B. B. et al., Amer. Soc. Plant Biol., pp. 1158-1203). Among the various abiotic stresses, drought, salinity and extreme temperature are major adverse environmental factors that limit crop productivity worldwide. Water stress in its broadest sense encompasses both drought and salinity or osmotic stress. The abiotic stresses of drought, salinity and freezing are linked by the fact that they all decrease the availability of water to plant cells. This decreased availability of water is quantified as a decrease in water potential. Plants resist low water potential and related stresses by various mechanisms such as the following: by modifying water uptake and loss to avoid low water potential; by accumulating solutes and modifying the properties of cell walls to avoid the dehydration induced by low water potential; and by using protective proteins and mechanisms to tolerate reduced water content by preventing or repairing cell damage. Salt stress also alters plant ion homeostasis (Verslues P. E. et al., (2006) Plant Journal, 45, 523-539; Koiwa, H. et al. (2006) J Exp Botany, 57(5):1119-1128; Rodriguez, M. et al. (2005) Biotecnologia Aplicada, 22:1-10).
Drought and salt stress, together with low temperature, are major problems for agriculture because these adverse environmental factors prevent plants from realizing their full genetic potential. Abiotic stress leads to a series of morphological, physiological, biochemical and molecular changes that adversely affect plant growth and productivity. Reviews on the molecular mechanisms of abiotic stress responses and the genetic regulatory networks of drought stress tolerance have been published (Valliyodan, B., and Nguyen, H. T., (2006) Curr. Opin. Plant Biol. 9:189-195; Wang, W., et al. (2003) Planta 218:1-14; Vinocur, B., and Altman, A. (2005) Curr. Opin. Biotechnol. 16:123-132; Chaves, M. M., and Oliveira, M. M. (2004) J. Exp. Bot. 55:2365-2384; Shinozaki, K., et al. (2003) Curr. Opin. Plant Biol. 6:410-417; Yamaguchi-Shinozaki, K., and Shinozaki, K. (2005) Trends Plant Sci. 10:88-94).
The application of transgenic technology, through either over- or under-expression of the transgenes, is a powerful tool for the engineering of stress-tolerant crop plants. Enhancement of resistance against abiotic stresses may also be achieved via traditional breeding, molecular breeding, and transgenic approaches.
There is a need for methods to identify plants resistant to realistic and reproducible stress conditions and which can be applicable to analyzing large numbers of lines. Such methods are important in order to develop more efficient screening procedures for germplasm evaluation and improvement of stress tolerance. Preferably, rapid and reliable screening procedures are desirable. Moreover, under natural conditions, plants rarely experience single stress factors one by one, but are much more likely to be exposed to multiple stresses simultaneously. Few studies have attempted to investigate plant responses under multiple stresses, either in the laboratory or the field. The response of plants to multiple stress combinations cannot always be extrapolated from responses to individual stress factors (Holopainen, J. K. et al. (2010) Trends Plant Sci. 15(3):176-184).